Fiber-reinforced composites generally include two primary components: a polymeric material composing the matrix material and a reinforcing material. The polymeric material may be a neat resin, a mixture, a blend, a compound or combination thereof. The polymeric material may include additives, fillers, stabilizers, pigments and/or other constituents. The reinforcing material may be a fiber which may include a sizing agent.
Typically, the polymeric material and the reinforcing material have different properties, such that when they are combined a composite material having intermediate properties is formed. For example, the polymeric material may be relatively low strength but may have relatively high elongation properties, while the reinforcing material may be very strong but relatively brittle. A composite plastic part derived from a composite material may have a strength that is greater than the polymeric material strength while also being relatively tough compared to the reinforcing material.
Injection molding is one of the most widely used processes for producing composite plastic parts. One common injection molding process utilizes pellets in finite lengths, which are prepared in advance and dispensed into an injection molding machine. Such pellets may be composite materials that include pre-impregnated fibers where a resin substantially wets out or impregnates each individual fiber also referred to herein as a filament. Alternatively, such pellets may include constituents of a composite material dosed together in the form of wire coated pellets where the resin encapsulates the outside of a mass of reinforcing fibers without substantial impregnation or wet-out, thereby requiring significant mixing within an extruder to achieve substantial wet-out and impregnation. Short fiber pellets include fibers randomly blended into a polymeric material (e.g., resin). Long fiber pellets may include unidirectional fibers. The length of the pellets may typically be limited to one (1) inch or less due to difficulties conveying and dispensing pellets of greater lengths. The pellets are fed into an extruder of an injection molding machine. At the same time, other types of pellets can be fed into the injection molding machine to introduce relevant functionalization to the resulting composite material. Other types of pellets include neat resin pellets, e.g., pellets that only include main identified polymers and optionally small fractional quantities of stabilizers and/or additives, and resin-containing pellets combined with additives and/or fillers. The pellets are introduced into the extruder in a cold, solid state to prevent the pellets from sticking to each other and/or blocking the feed into the extruder. After dispensing pellets into the extruder, the pellets are subjected to considerable shear by the extruder to heat and melt the resin as the pellets are also blended and homogenized into a uniform compound for forming (e.g., molding or extrusion). The shear, solid-melt interface and/or frictional forces during this process (plastification) reduces fiber length significantly.
An alternative form of injection molding includes an in-line compounding method where dry tows of glass reinforcing fibers (e.g., not wetted by any polymers, other than potentially a sizing) are exposed to resin and/or introduced into a melt stream of resin requiring significant mixing within an extruder to achieve substantial wet-out and impregnation. In this process, the fiber is dispersed within and wet-out by the resin and cut if necessary followed by blending, mixing and/or homogenization into a compound of uniform density for injection into a mold to produce injection molded parts.
Compression molding is another process used to produce plastic parts where a molding compound is placed in an open temperature-controlled mold cavity avoiding the runner system typically required by injection molding and reducing the shear exposure of the fibers. The mold is closed and pressure is applied to force the molding compound into all areas of the mold cavity. During this step, temperature and pressure are maintained to cure a thermoset molding compound or solidify a thermoplastic molding compound. Compression molding can be used to mold intricate and high-strength fiber reinforced parts. Materials that comprise the molding compound can be loaded into the mold either in the form of thermoplastic flakes (which must be melted within and subsequently cooled by the mold), an extrudate of bulk thermoset or thermoplastic molding compound, sheets of thermoset or thermoplastic molding compound, or combinations of formats.
An alternative form of compression molding includes an inline compounding option known in the industry known as LFT-D (Long Fiber Thermoplastic—Direct). Again, the composite plastic molding compound in the form of an extrudate is produced in-line in which dry tows of glass reinforcing fibers are exposed to resin and/or introduced into a melt stream of resin requiring significant mixing within an extruder to achieve substantial wet-out and impregnation. The fiber is dispersed within and wet-out by the resin and cut if necessary by blending, mixing and/or homogenization into a uniform compound for molding. In LFT-D, the molten compound is introduced directly into the compression mold.
Various extrusion processes are also utilized to directly produce plastic parts. Such processes can be used to extrude a shaped product from a molding compound. Extrusion is used to create parts with a fixed cross-sectional profile. A composite or polymeric material in pellet form is fed into an extruder that heats and softens the composite or polymeric material. The molding compound is pushed out via a die, possibly into cool water that solidifies the extruded product. Extrusion can be utilized to create very complex cross-sections having excellent surface finishes.
Blow molding is another process used to produce plastic parts. Blow molding is used to manufacture hollow plastic parts. The blow molding process typically begins with preparing a molding compound from pellets and forming it into a parison. The parison is a tube-like piece of molten material with a hole in one end through which compressed air can pass. The parison is then clamped into a mold and air is blown into it. The air pressure then pushes the molten material out to match the mold. Once the molten material has cooled and hardened the mold opens and the plastic part is ejected.